The two-cycle engine has a piston making a linear reciprocating motion by combustion of a mixed gas, a crankshaft outputting a rotational motion, and a connecting rod connecting the piston and the crankshaft to each other to convert the linear reciprocating motion to a rotational motion.
The connecting rod has a large-end portion at a lower portion of a straight rod and a small-end portion at an upper portion thereof. The crankshaft and a piston pin coupling the piston and the connecting rod to each other are rotatably supported at the large-end portion of the connecting rod and at the small-end portion of the connecting rod respectively via roller bearings mounted on engaging holes respectively. The roller bearing supporting the rotational shaft is constructed of a plurality of rollers and a cage retaining the rollers.
The roller bearings, mounted on the engaging holes formed at the small-end portion of the connecting rod and the large-end portion thereof, which support the piston pin and the crankshaft respectively are capable of receiving a high load, although a projected area thereof is small. As the roller bearing, a needle roller bearing having a high rigidity is used. The needle roller bearing includes a plurality of needle rollers and a cage retaining a plurality of the needle rollers. The cage is provided with pockets for retaining the needle rollers respectively. A columnar portion positioned between the pockets retains the interval between the needle rollers. To decrease a load applied to the needle roller bearing owing to rotations of the needle rollers and rotations thereof on the center of the cage, the needle roller bearing disposed at the small-end portion of the connecting rod and the needle roller bearing disposed at the large-end portion thereof are used by guiding an outside-diameter surface of the cage, namely, by positively bringing the outside-diameter surface of the cage into contact with the inside-diameter surface of the engaging hole formed at the small-end portion of the connecting rod and the large-end portion thereof.
On the other hand, in the ordinary rolling bearing, the inside thereof is sealed with an inner ring, an outer ring, and a sealing member. The inside of the bearing is provided with rolling elements and a cage. Grease is filled inside the bearing. The rolling elements and the cage are always lubricated with the grease. On the other hand, because the above-described needle roller bearing is provided with none of the inner ring, the outer ring, and the sealing member, the inside of the bearing is not sealed, and the grease cannot be filled inside the bearing. Therefore when the needle roller bearing rotates, it is necessary to always supply lubricating oil to a sliding portion by a pump or the like.
Because the pump or the like starts to operate simultaneously with a start of the rotation of the needle roller bearing, the lubricating oil does not spread to the entire needle roller bearing immediately after the needle roller bearing starts to rotate. Thus a sufficient lubrication is not accomplished. Therefore friction is generated to a high extent between the cage and the needle roller. Thereby wear occurs on the surface of the cage and that of the needle roller, and on the outside-diameter surface of the cage and the inner-diameter surface of the housing of the bearing. In the worst case, there is a fear that both seize on each other.
Therefore to prevent wear and seizing which occur immediately after the needle roller bearing starts to rotate, an art of forming a film having lubricating property on the surface of the cage in advance is proposed.
For example, a method of forming a hard film of diamond-like carbon (hereinafter referred to as DLC) on the guide surface of the rolling elements of the cage made of the steel material having the hard layer formed on the surface thereof by a carburizing process by using a sputtering method and thereafter forming a film of a soft metal such as silver on the film of the DLC (see patent document 1) is known.
According to the description made in the patent document 1, the film of the soft metal decreases the friction between the cage and the needle roller and the friction between the outside-diameter surface of the cage and the inside-diameter surface of the housing. Therefore it is possible to prevent the seizing of the cage and the needle roller even at the time immediately after the start of the rotation although the lubrication is insufficient at this time. Further even though the film of the soft metal wears with its use, the DLC film disposed under the film of the soft metal is newly exposed and prevents the wear.
An art of directly forming the film of the soft metal on the surface of the cage by a plating method is proposed. For example, a method of forming a silver-plated film having a thickness of 25 to 50 μm on the surface of low carbon steel is known (see patent document 2). According to the description made in the patent document 2, the silver-plated film decreases the friction between the cage and the needle roller and the friction between the outside-diameter surface of the cage and the housing. Therefore as described above, the occurrence of seizing can be prevented even at the time immediately after the rotation starts in an insufficient lubrication. Similarly to the silver-plated film, the copper-plated film has an operation of decreasing the friction between the cage and the needle roller. Thus the copper-plated film is capable of preventing seizing.
But in the method described in the patent document 1, after the soft metal wears and disappears, the hard film is exposed, and the inside-diameter portion of the housing slides on the hard film. In this case, although the cage does not wear, there is a fear that the inside-diameter portion of the housing is worn by the hard film formed on the surface of the cage. From the standpoint of the production, because the carburizing processing is carried out on the cage, the DLC film is formed by a sputtering apparatus, and the soft metal film is formed, operation steps are complicated and many steps are required. Further the sputtering apparatus is expensive and provides an unfavorable production efficiency. Therefore the processing to be performed by using the sputtering apparatus costs high.
In the method described in the patent document 2, in the lubricating system containing the sulfur-based additive, the silver-plated film formed on the surface of the cage binds with the sulfur component contained in the lubricating oil to form silver sulfide. The silver sulfide coats the surface of the silver-plated film. Because the silver sulfide is more frail than silver, the film peels or is inferior in its oil resistance. Thus the film is dissolved in the lubricating oil. Consequently the friction between the outside-diameter surface of the cage from which the silver-plated film disappears and the inside-diameter surface of the housing increases and thus seizing is easy to occur. Similarly, copper sulfide is formed from the copper-plated film, which poses a problem that owing to the peeling and dissolution of the film, the lubricating property of the cage deteriorates.
The present inventors proposed a technique of forming a resin film containing fullerene on the surface of a cage (patent document 3). The resin containing the fullerene is capable of forming a lubricating film excellent in its wear resistance.
But the behavior of the wear-resistant lubricating film containing the fullerene in a lubricating system or atmosphere containing a sulfur-based additive is unknown.
Patent document 1: Japanese Patent Application Laid-Open No. 2005-147306
Patent document 2: Japanese Patent Application Laid-Open No. 2002-195266
Patent document 3: Japanese Patent Application Laid-Open No. 2005-299852